Bonding to Existing Concrete

posted by Bob

Fact: Fresh wet concrete does not normally bond well to existing dry concrete. Do you remember elementary school where one of the subjects on which you were graded was “plays well with others”? Concrete would have gotten an F. There is nothing in basic portland cement that will act as a bonding agent. Portland cement concrete works well in mass and provides great compressive strength but not bond.

Concrete is marvelous stuff but in time it will deteriorate. When it does, you either have to patch it or replace it. Assuming that it is structurally sound the least expensive alternative is to patch it. However patching it requires some attention to detail or your patch will not last. So that you don’t waste too much time or money, we should probably discuss what “structurally sound” means. If your sidewalk has either heaved or dropped at almost every joint, repairing it will not provide a long-term solution. The slabs are likely still moving. If your slab has so much sand and gravel on the surface that despite sweeping and sweeping and squirting and squirting it just keeps coming back, don’t waste your time on repairs. If you have multiple cracks that run so deep that they appear to run through the slab, a repair would only be temporary. The solution to all of these problems involves a jackhammer and bags of one of the Sakrete concretes.

Since this discussion is on the best way to bond concrete, we will assume that your slab is good.

There are a variety of Sakrete concrete repair products available to fix concrete that has begun to deteriorate. However, without good surface preparation, none of them are going to perform satisfactorily. All loose sand, gravel, dirt, leaves etc. must be removed. This can typically be done with a garden hose and a good nozzle. Tough areas may require a pressure washer or mechanical abrasion. The two toughest areas to cover are those with oil and tree sap. Both of these will work their way down into the concrete. Simply washing the surface isn’t sufficient. If the stains do not run too deep, you can chip away the concrete using a hammer and chisel. Don’t forget the goggles (not just glasses) as this process will throw concrete all over the place. Also, keep your thumb out of the way. If the spots are too large or too deep for this to be practical, you may need a sealer to cover the stains before patching.

There are two basic methods for bonding a portland cement based product to existing concrete; 1) chemically and 2) mechanically.

Let’s discuss the mechanical approach first since it is really used in both approaches. The most effective way to ensure a really good bond is with a scratch coat. This is simply a very wet coat made up by mixing the repair product with water. Mix up a small amount of the repair material to a soupy consistency. You don’t need to measure the water-just turn the stuff into slop. Then, using a gloved hand or a rag, smear the material onto the area to be patched. Just think finger painting from kindergarten. The technique is about the same. Apply pressure to ensure that as much as possible is shoved into the nocks and crannies. You only need a thin coat. It is not necessary for this scratch coat to dry. By the time you get the repair material mixed it will be ready. Then mix up additional repair material to the proper consistency and apply over this thin scratch coat.

The chemical approach involved mixing up a liquid bonding agent that helps bond new concrete products to old. Products like Sakrete Top'n Bond and Sakrete Flo-Coat already contain polymers that greatly improve the bond of portland cement and should NEVER be used with a liquid bonding agent. I know in America bigger is better but it’s just not so with these products. Other products like Sakrete Sand Mix and Sakrete Fast Set Cement Patcher benefit from the use of a liquid chemical bonding agent such as Sakrete Bonder/Fortifier. When using a liquid bonding agent, paint the bonder onto the existing concrete and allow it to dry until it is tacky. This usually takes only a few minutes. Then apply the repair material. Just as in the process described above, after the bonder has become tacky apply a scratch coat and then apply the repair material. The most effective way to ensure that the bonding agent gets into the existing concrete is to apply it directly using a brush or rag. It can be sprayed if you happen to have a sprayer. Although the directions say that you can use it as part of the mix water, direct application works better.

If you are doing a large area and a scratch coat isn’t practical you will need to spray the surface with water before you apply the repair material. On a warm day, the existing concrete surface will be hot enough to suck the water out of the repair material. In addition, some concretes are quite porous and will rob water from your repair material. If too much water is lost into the old concrete there will not be enough water to hydrate all of the cement particles and a lower strength material will be the result. Concrete simply will not bond to all substances. Paint, oil, glue from old flooring tiles are just a few. You must mechanically remove these materials if you want the job to last.

Once the job is complete, you can do a quick check to see if the bond was successful. Wait at least 24 hours and then tap “gently” on the patch using a hammer or some other dull object and listen for a hollow echoing sound. If you just get a dull thud then the material has bonded well. If you get a hollow sound, the material has not bonded and will crack in time. Which means it is back to the beginning of today’s topic. Here is hoping your concrete work comes across as a dull thud (not like some of my party guests) rather than a hollow endeavor.



I have a 2ft wide slab I need to extend about 4" in one direction and is 4-6" thick. There will be a horse waterer on top with even weight distribution. Dirt will be packed tight around it and covering it and around waterer about 2" high when complete. The concrete will only extend about 1.5-2" around the unit. Will making a form and pouring work or do I need to attach it somehow?
- Jessica
Saturday, November 21, 2015 at 1:19 PM

Eric, Sakrete Sand Mix modified with Sakrete Bonder and Fortifier (as a bonding agent/primer) is recommended for your project. It is necessary that you properly prep the area so that it is free of any bond breakers such as dirt, debris, loose material, etc. A mechanical bond can be achieved by anchoring/epoxying rebar to the existing substrate. Sakrete Bonder and Fortifier should then be brush on the surface and also used as an admixture 1:1 with water to the Sand Mix. AVOID a soupy mix.
- Chris Technical Services
Tuesday, November 17, 2015 at 3:33 PM

Hill, you must first start by properly preparing the surface that you are going to be applying the new material to by cleaning off any dirt, debris, loose material, paint, stain, oil, etc. Sakrete Top'n Bond if polymer modified and can be used up to 1/2" down to a feather edge. Multiple lifts can be applied to achieve a proper slope. When the first lift is thumbprint hard you may apply the next lift.
- Chris Technical Services
Monday, November 16, 2015 at 3:30 PM

Hello, I am installing limestone on a basement walk out set of stairs. The concrete work for the stairs was not done corecctly so I could make even size riser heights for the stairs. I corrected all the stairs except the top one that sits on the foundation wall and is about 2 1/2 inches too low. So I need to raise it by about 2 inches and then mortar my limestone to it. The step I am trying to raise is concrete block with it's holes filled with concrete that have been that way for five years. I was hoping for advice on the best way to do this so it will not fall apart. I am more concerned about it lasting long term so if I can do anything more to have better results please include any ideas. Thank You for your help, Eric
- Eric
Monday, November 16, 2015 at 2:12 PM

What can I use to level inside a polymer concrete ACO drain which runs in front of my garage. The slope of the drive means that inside the ACO drain needs to be raised at one end by half an inch and by one and a half inches at the other end in order for water to flow correctly. At present water is lying at one end.
- Hill
Monday, November 16, 2015 at 11:46 AM

Arthur, yes, Sakrete Sand Mix modified with Sakrete Bonder and Fortified (as a bonding agent/primer and admixture) may be the best products to use. You must first properly prep the area by removing any bond breakers such as dirt, dust, debris, loose material, paint, stain, oil, etc. Once the area clean, dry, and solid sound substrate, brush a thin layer of Sakrete Bonder and Fortifier using an inexpensive paint brush to the area. Sand Mix can be used from 2” down to ½”. From ½” to a feathered edge Sakrete Flo Coat is recommended due to its flowable nature and ease of use. Flo Coat can be applied to the whole area for a uniform look. Do not use Bonder and Fortifier with Flo Coat.
- Chris Technical Services
Monday, November 9, 2015 at 9:33 AM

My concrete "porch" is 14 feet wide by 40 feet long and covered. When they originally laid the concrete they sloped it gradually from one end to the other to shed the water. I am having it covered and would like to eliminate the slope. It is fairly level until about 10 feet from the lower end and has a slight indention at the lower end. I would like to level the concrete without having to remove it all. Is there a product that will adhere to the concrete that appears to be a couple of inches lower than the level part.
- Arthur
Thursday, November 5, 2015 at 1:38 PM

L.Steffen (Lisa), yes, this can be done by using Sakrete Type S Mortar/Stucco Mix along with Sakrete Concrete Bonder and Fortifier. There are a few steps that you should be aware of. First check with your local building code to ensure you are meeting all requirements. Second, properly prep the surface by removing any bond breakers such as dirt, debris, paint, stain, oil, etc. Once you have a clean sound surface apply the Sakrete Concrete Bonder and Fortifier to the surface undiluted as a primer where you will be installing your mortar bed. Use Sakrete Type S Mortar/Stucco Mix as a mortar bed for your block and level starting in a corner. The block should have 3/8” mortar joints between them. You can view a demonstration on our website under project videos.
- Chris Technical Services
Monday, November 2, 2015 at 10:31 AM

I have a 10x8 wood garden shed with concrete floor. The wood siding is old and starting to rot along lower boards due to being in contact with ground. I would like to reside but was wondering if it is possible to bond concrete cinder blocks, maybe 3 high then reside with cedar wood but only to the top of the cement blocks so the wood does not contact ground. Can I bond cinder block to an already existing concrete slab? The slab is in great condition yet. Any advice would be helpful. thank you. Lisa
- L.Steffen
Sunday, November 1, 2015 at 4:49 PM

Gregory, unfortunately we do not have a recommended product in our product line that we suggest for the construction of your pool.
- Chris Technical Services
Tuesday, October 27, 2015 at 2:10 PM



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